The present invention relates to thermoplastic perhalogenated polymers having a combination of improved mechanical properties.
Specifically the present invention relates to chlorotrifluoroethylene (CTFE) thermoplastic polymers modified with perfluoroalkylvinylethers, having a combination of improved mechanical properties, specifically at room temperature an improved combination of stress at break and strain at break without substantially affecting the yield stress, in comparison with the CTFE (PCTFE) homopolymer.
The polymers of the invention are to be used for obtaining manufactured articles such as pipes, sheets, films having the above mentioned characteristics.
It is known in the prior art that PCTFE is a fluorinated resin having excellent chemical resistance with good impermeability properties to gases and vapours, more specifically to oxygen, nitrogen and water vapour, but mechanical properties typical of a brittle material, i.e. high values of elastic modulus and yield stress combined with low values of stress and strain at break. As known the PCTFE polymers having a high viscosity, i.e., low Melt Flow Index (MFI) and high molecular weight show better mechanical properties than those having a low viscosity, i.e., high Melt Flow Index and low molecular weight. However the use of PCTFE having a very high viscosity is extremely difficult or unfeasible in the fluorinated thermoplastic processing for obtaining manufactured articles, such as pipes, sheets and films. Especially in the film processing, where the PCTFE for its characteristics is widely used in multilayer systems in food and pharmaceutical packaging products.
U.S. Pat. No. 5,145,925 describes a CTFE copolymer with a perfluoroalkylvinylether in an amount between 0.01 and 1% by moles; the preferred comonomer is perfluoropropylvinylether (FPVE). The all exemplified FPVE/CTFE copolymers, those with low (0.05% by moles) and high content (1.1% by moles) of modifying agent, show stresses at break lower than those of the homopolymer. In addition, the described copolymers containing less than 1% by moles of FPVE show elongations at break lower than 100%. The said elongation improves with the FPVE increase and the yield stress diminishes. Therefore a global improvement of the mechanical properties of the exemplified polymer in comparison with the CTFE homopolymer is not described.
The need was therefore felt to have available a thermoplastic perhalogenated PCTFE having besides the intrinsic homopolymer characteristics (chemical resistance and impermeability) a combination of mechanical properties of improved stresses and elongations at break and without substantially decreasing the yield stress in comparison with those of the homopolymer.
The Applicant has unexpectedly and surprisingly found that by the use of specific comonomers as hereinunder specified it is possible to obtain CTFE thermoplastic copolymers having an optimal combination of mechanical properties and characterized by a higher stress at break than those of the homopolymer and of the copolymers described in the prior art at a given MFI and modifier content.
An object of the present invention are therefore chlorotrifluoroethylene (CTFE) thermoplastic perhalogenated polymers, formed by the following polymer components:
(I) 10-90% by weight of a CTFE homopolymer and
(II) 90-10% by weight of a CTFE copolymer modified with comonomer (a) selected from one or more perfluoroalkyl vinylethers having the formula:
Rfxe2x80x94Oxe2x80x94CFxe2x95x90CF2
wherein:
Rf is a C2-C5 perfluoroalkyl group, resulting in the final polymer (I)+(II), the amount of (a) being from 0,2 to 5% by moles and the CTFE the remaining part to 100% by moles.
Among the polymers of the invention, those having a RMFI lower than 0.3 or higher than 3, preferably lower than 0.2 or higher than 5, more preferably lower than 0.05 or higher than 20, are preferred.
With RMFI it is meant the Melt Flow Index ratio (MFI, measured at 265xc2x0 C. and 10 kg of load according to the ASTM D 1238-88 method) of the polymer component (I) and of the final polymer (sum of the polymer components I and II).
The polymers of the invention having a RMFI lower than 0.3, preferably lower than 0.2, more preferably lower than 0.05 characterized in that the polymer component (II) contains more than 1% by moles of the perfluoroalkylvinylether of comonomer (a), are more preferred.
Among the perfluoroalkylvinylethers of formula (a) the perfluoroethylvinylether and the perfluoropropylvinylether (PPVE) are preferred.
The polymers object of the invention are CTFE thermoplastic polymers having a MFI higher than 0.1 g/10xe2x80x2, preferably higher than 0.5 g/10xe2x80x2, more preferably higher than 2 g/10xe2x80x2.
The polymer component (I) of the polymer of the invention preferably ranges from 30 to 70%, more preferably from 40 to 60% by weight.
The polymers of the invention can be obtained by blending powder or latex of components (I) and (II).
The polymers (components I and II) can be prepared by (co)polymerization of the corresponding monomers, in suspension in organic medium or in water or in aqueous emulsion in the presence of a radical initiator, at a temperature generally in the range xe2x88x9220xc2x0 C.-150xc2x0 C., preferably 10xc2x0 C.-100xc2x0 C., more preferably 10xc2x0 C.-70xc2x0 C. The reaction pressure is generally in the range 1.5-80 bar, preferably 3-37 bar, still more preferably 4-26 bar.
As initiator any compound able to generate active radicals at the selected (co)polymerization temperature can be used. The various radical initiators can be selected from:
(i) bis-acylperoxides of formula (Rfxe2x80x94COxe2x80x94O)2 wherein Rf is a C1-C10 (per)haloalkyl (see for example EP 185,242 and U.S. Pat. No. 4,513,129), or a perfluoropolyoxyalkylene group (see for example EP 186,215 and U.S. Pat. No. 5,021,516); among them, bis-trichloroacetylperoxide and bis-dichlorofluoroacetylperoxide (see U.S. Pat. No. 5,569,728), are particularly preferred;
(ii) inorganic peroxides soluble in water, such as monovalent cation persulphates or perphosphates; sodium and potassium persulphates are particularly preferred;
(iii) organic or inorganic redox systems, such as potassium persulphate/sodium sulphite, terbutylhydroperoxide/methabisulphite (see U.S. Pat. No. 5,453,477).
In the case of the suspension (co)polymerization, the reaction medium is formed of an organic phase, to which water is sometimes added in order to favour the heat dispersion which develops during the reaction. The organic phase can be formed of the monomers themselves, without addition of solvents, or of the monomers dissolved in a suitable organic solvent. As organic solvents chlorofluorocarbons, such as CCl2F2 (CFC-12), CCl3F (CFC-11), CCl2FCClF2 (CFC-113), CClF2CClF2 (CFC-114), etc. are conventionally used. Since such products have a destroying effect on the ozone present in the stratosphere, alternative products, such as the compounds containing only carbon, fluorine, hydrogen, and optionally oxygen, described in U.S. Pat. No. 5,182,342, have been proposed. In particular fluoropolyethers with at least one hydrogenated end group, preferably two, of the xe2x80x94CF2H, xe2x80x94CF2CF2H, xe2x80x94CF(CF3)H type, can be used.
The usable amount of the radical initiator is the standard one for the (co)polymerization of fluorinated olefinic monomers, and is generally in the range 0.003%-10% by weight with respect to the total amount of (co)polymerized monomers.
In the case of the emulsion (co)polymerization, the reaction is carried out in the presence of a suitable surfactant, see for example those described in U.S. Pat. Nos. 4,360,652 and 4,025,709, so as to give to a stable emulsion. They are generally fluorinated surfactants, selected from the compounds of general formula:
Rfxe2x80x2xe2x80x94Xxe2x88x92M+
wherein Rfxe2x80x2 is a C5-C14 (per)fluoroalkyl chain or a (per)fluoropolyoxyalkylene chain, Xxe2x88x92 is xe2x80x94COOxe2x88x92 or xe2x80x94SO3xe2x88x92, M+ is selected from: H+ and one alkaline metal ion. Among those commonly used we remember: sodium perfluoro-octanoate; (per)fluoropolyoxyalkylenes ended with one or more carboxylic groups; sulphonic acid salts of formula Rfxe2x80x3xe2x80x94C2H4SO3H, wherein Rfxe2x80x3 is a C4-C10 perfluoroalkyl (see U.S. Pat. No. 4,025,709); etc.
Preferred surfactants are those, wherein M+ is selected between Na+ and K+, preferably K+, which, when combined with inorganic initiators of Na+ and K+, preferably K+, give non-discolored polymers.
The preferred methods to control the molecular weights of the polymers, components (I) and (II), are:
Ixe2x80x2) Control the concentration of the free radicals in the reaction medium; an increase of the free radical concentration during the polymerization determines a viscosity decrease (MFI increase) and viceversa;
IIxe2x80x2) Control the polymerization temperature; a temperature increase determines a viscosity decrease (MFI increase) and viceversa;
or their combination.
To obtain a high molecular weight (low MFI), a low synthesis temperature (10xc2x0 C.-30xc2x0 C.) combined with a low concentration of free radicals coming from the initiator, is preferred; viceversa to obtain a low molecular weight (high MFI), an high synthesis temperature (60xc2x0 C.-80xc2x0 C.) combined with an high concentration of free radicals coming from the initiator, is preferred.
When chain transfer agents for obtaining low molecular weights are used, these can preferably be halogenated hydrocarbons, for example chloroform or HCFC 123 and ethane or methane. The transfer agent amount can range within rather wide limits, depending on the reaction temperature and the molecular weight target. Generally, such amount ranges from 0.001 to 5% by weight, preferably from 0.05 to 1% by weight, with respect to the total amount of monomers introduced to the reactor.
The process object of the present invention can be advantageously carried out in the presence of perfluoropolyoxyalkylene dispersions, emulsions or microemulsions, according to U.S. Pat. Nos. 4,789,717 and 4,864,006, or also of fluoropolyoxyalkylene microemulsions having hydrogenated end groups and/or hydrogenated repeating units, according to U.S. Pat. No. 5,498,680.
The polymerization can also be carried out by using a radical photoinitiator in the presence of visible ultraviolet radiation, either in suspension in organic medium or in water or in emulsion/microemulsion, according to what described in European patents EP 650,982 and EP 697,766 in the name of the Applicant, herein incorporated by reference.
From an operating point of view, thermally stable photoinitiators at the polymerization temperature and also at room temperature are preferred, and among them, perhalogenated organic peroxides or inorganic such as potassium persulphate or sodium persulphate are particularly preferred.
The preferred process according to the present invention is carried out in (per)fluoropolyoxyalkylene microemulsion, wherein the surfactant is a Na+ or K+ salt, preferably K+, and in the presence of an inorganic Na+ or K+ initiator, preferably potassium persulphate.
The preferred preparation process of the polymers of the invention is the polymerization of component (I) or component (II) in presence of component (II) or component (I) respectively, as above defined both components (I) and (II). The most preferred preparation process of the polymers of the invention is the synthesis of component (II) in presence of component (I), as above defined.
As already said, the polymers of the present invention are used for preparing manufactured articles, specifically pipes, sheets and films.
The second melting temperature (T2f) and the crystallization temperature (TXX) are determined by differential scanning calorimetry (DSC).
The monomer composition has been determined by 19-F-NMR.
The mechanical properties, obtained according to ASTM D 1708, using compression-molded specimen, are reported in Table.
The discoloration is determined through the observation of the strands obtained after 20, 40 and 60 minutes of residence time of the polymer in the MFI machine at 265xc2x0 C. Discoloration means the coloration of the extruded polymer. The product not showing discoloration is colourless or white. Polymers showing discoloration are generally yellowish or brown. The skilled in the art is able to determine if the polymer shows or not discoloration, also using methods measuring the colour index such as for example the white or yellow index.
The following examples are given for illustrative purposes and are not limitative of the present invention.